Re: [PATCH v2] powerpc/pseries/svm: Allocate SWIOTLB buffer anywhere in memory

[Thiago Jung Bauermann <bauerman@linux.ibm.com>]

Christoph Hellwig <hch@lst.de> writes:

> On Mon, Aug 17, 2020 at 06:46:58PM -0300, Thiago Jung Bauermann wrote:
>> POWER secure guests (i.e., guests which use the Protection Execution
>> Facility) need to use SWIOTLB to be able to do I/O with the hypervisor, but
>> they don't need the SWIOTLB memory to be in low addresses since the
>> hypervisor doesn't have any addressing limitation.
>> 
>> This solves a SWIOTLB initialization problem we are seeing in secure guests
>> with 128 GB of RAM: they are configured with 4 GB of crashkernel reserved
>> memory, which leaves no space for SWIOTLB in low addresses.
>> 
>> To do this, we use mostly the same code as swiotlb_init(), but allocate the
>> buffer using memblock_alloc() instead of memblock_alloc_low().
>> 
>> We also need to add swiotlb_set_no_iotlb_memory() in order to set the
>> no_iotlb_memory flag if initialization fails.
>
> Do you really need the helper?  As far as I can tell the secure guests
> very much rely on swiotlb for all I/O, so you might as well panic if
> you fail to allocate it.

That is true. Ok, I will do that.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center